1. Field of the Invention
This invention relates to dewatering sludges, particularly sludges derived from food preparation. It also relates to processing such sludges for use as biomass fuels.
2. Review of the Prior Art
In fish canneries, red meat slaughter houses, and poultry slaughter and processing plants, large amounts of food wastewater, having high protein and fat contents, are produced. A poultry processing plant is believed to be typical of food preparation plants that produce large quantities of wastewater containing fat and protein. Using such a plant producing 500,000 gallons per day of wastewater as an example, the fat content of the wastewater is in its insoluble biochemical oxygen demand (BOD) fraction, and a greater part of the protein content is in its soluble BOD fraction. The fats are in the form of particles that would be separated from the wastewater in a filtration operation. Over an extended period, the insoluble fraction is fairly constant, but the soluble fraction is subject to wide variations caused by such routine process situations as emptying a scalder or a cook vat.
These meat wastewaters are commonly processed by: (a) pretreatment of the wastewaters in a float cell by use of a dissolved air flotation process which is operated with or without chemical coagulants and flocculants and which produces a primary float sludge or skimmings and a partially purified wastewater and /or (b) biological oxidation of the meat wastewaters or of the partially purified wastewater under facultative or aerobic conditions in an aerobic treatment plant, such as a barrier oxidation ditch, or biological reduction or fermentation under anaerobic conditions to produce biological sludges.
Biological sludges are defined herein as those sludges produced by conversion of carbonaceous, proteinaceous, and fatty matters and/or sugars and carbohydrates to biological solids (microorganisms including bacteria, fungi, protozoa, rotifers, and the like) by biological oxidation under facultative or aerobic conditions to create activated sludge or by biological reaction or fermentation under anaerobic conditions to create anaerobic sludge. Biological sludges include municipal or sewage sludge which is relatively easy to dispose of by land application or by drying and landfilling.
It should be understood that all of the wastewater can be fed to the oxidation ditch, but the very high biological oxygen demand (BOD) created by fats makes it desirable to remove the fats by treating the wastewater in a flotation cell which produces a float sludge and a purified wastewater which is fed to the oxidation ditch. If no chemicals are employed in the float cell, a "roughing sludge" is produced; it contains a portion of the fat and a sufficient portion of the protein that it is subject to the same wide variations as the wastewater itself.
Addition of chemical coagulants and flocculants in the dissolved air flotation process is preferred because these additives float additional proteins from the wastewater. When chemicals (coagulation agents and flocculation agents) are employed, nearly all (98%) of the insoluble fraction and a variable portion of the soluble fraction, depending upon the nature and amount of the added chemicals, are separated in the chemical float sludge that is produced. Without such chemicals, the float sludge has a fat content of up to 75% by weight and more on a dry basis; with the chemicals, the protein content is increased and the weight percentage of fat is lowered, but recoveries vary greatly between plants.
The chemical float sludge produced by dissolved air flotation with added chemical coagulants and flocculants from wastewater produced by pure poultry slaughtering operations, for example, is a complex substance which contains approximately 30-40% protein, 5-12% ash and fiber, and 40-50% fat on a dry basis and has a solids content ranging from 5% to 30% but normally averaging about 10-15%.
In a pure further processing poultry plant, particularly if it contains a bread line, there is so much flour and so little protein in the wastewater that the protein content of its float sludge may be as low as 3% by weight. Generally, float sludge produced by such a further processing process has 5-20% protein, about 5% ash and fiber, and 40-65% fat, depending upon use of oil frying and the like. Further processing is now being incorporated increasingly in poultry slaughtering plants.
The high fat content complicates the disposal process and increases its cost. A poultry slaughtering plant handling 70,000 birds per day (17,500,000 birds per year) produces 2,200-2,500 gallons per day (5000,000 gallons per year) of this float sludge which typically has a 12-15% solids content and costs $0.02-0.05 per gallon for disposal. This yearly disposal cost for float sludge is therefore $11,000 to $27,500 per year.
A poultry processing plant of this size additionally produces activated biological sludge from aerobic processing, such as in an oxidation ditch, of its partially purified wastewater which is fed from its float cell after skimming off the float sludge. A portion of the activated sludge is returned to the aerobic treatment basin (e.g., a barrier oxidation ditch), and the remaining portion (the waste activated sludge, WAS) is thickened in a suitable manner from 1-2% solids content to 6-7% solids content. The thickened WAS is generally sent to a sludge disposal site.
Because poultry processing plants are generally run on a single-shift, five-day-per-week basis, the chemical float sludge (CFS) is also produced on this basis, but the waste activated sludge (WAS) is produced seven days per week in the amount of approximately 4,000 gallons per day at 1% solids and 1,300-800 gallons per day of thickened sludge at 3-5% solids after passage through a flotation thickener.
In some plants, the WAS is stored from Monday through Friday and is then thickened over the weekend in the same float cell in which the raw wastewater is treated during the work week. Before thickening, it is similar to municipal or sewage sludge in many respects, such as having a low solids content of 2-8% by weight.
If this float sludge is hauled in a tank truck to a land disposal site, spread on the land, and plowed under, the disposal cost is $30 to $50 or more for each 2,000 gallons of activated sludge. The annual cost of disposing of this thickened activated sludge is $4,300-$7,200. Thus the combined costs of disposing of both float and activated sludges are $16,300 to $34,700 per year.
In a poultry processing plant, a rendering plant is used to process feathers, heads, feet, and other separated parts of the poultry carcass, characterized as "offal", into animal feed by "cooking" the material in a rotating drum-type dryer heated by steam in its shell to a temperature of about 240.degree. F. Such a dryer, known as a Duppes cooker, contains paddles, aligned at an angle to its axis, that push material from one end to the other. Float sludge is sometimes admixed with offal to produce a feed mixture for the rendering plant, but the mixture creates a gummy coating in the cooker, adds free fatty acids to the rendering products, and discolors these products.
As a method for compensating for surges in wastewater flow rate and in the insoluble fraction thereof and especially for equalizing dumps and spills from the rendering plant, it is possible to provide a sufficiently long equalization time by means of a holding tank within which the flow of float sludge can accumulate. For a poultry processing plant producing 2,500 gallons per day of float sludge, a 20,000 gallon holding tank would be required. Disadvantages of this method, however, include the high capital cost of the large tank and odor production; in summer, odor can develop within 24 hours. Accordingly, another and more practical surge-compensation method must be provided.
In co-pending Application Ser. No. 06/677,556, now U.S. Pat. No. 4,728,517 a process is provided in which float sludge, with or without activated biological sludge, is dewatered and defatted to provide a proteinaceous meal, which is suitable for animal food, and fat-impregnated chips, which are an excellent fuel source. Such chips can be readily burned in the cyclone furnace described in U.S. Pat. No. 4,612,865 which comprises a generally cylindrical and horizontal combustion chamber closed at one end and having at least one inlet opening for fuel and air, an outlet at the opposite end, and an ash discharging opening at its bottom and near its end wall. An ash discharge conduit conducts ash to an ash bin.
Such chips can also be burned by using the pressurized cyclonic combustion method and the cylindrical burner apparatus disclosed in U.S. Pat. No. 4,671,192 which produces a clean pressurized effluent gas. In the burner, particulate solids such as wood chips are fed tangentially into a primary combustion chamber at its inlet end and flow at high tangential velocity in a helical path through the burner.
A useful burner head for combusting solid fuels such as straw, peat, chips, and coal is disclosed in U.S. Pat. No. 4,546,710. The burner head comprises two pipes located concentrically to each other, the gap between the pipes being in communication with a supply source for combustion air and in communication with the inner pipe via two radially arranged sets of apertures in the wall thereof. The first set is located a sufficient distance from the charging opening to form a first combustion zone in which the fuel is incompletely burned, and the second set is located closer to the discharge opening to form a second combustion zone in which gases fromthe first zone are burned completely.
Even though these burners are satisfactory for burning the fat-impregnated chips produced by the process of co-pending Application Ser. No. 06/677,556, now U.S. Pat. No. 4,728,517 they are economically unsatisfactory because they are too big to fit into the standard boiler shell for bilers of less than 700 horsepower and especially of 250-400 horsepower and consequently have to be fitted into boilers constructed on site. For a 400-horsepower installation, for example, the costs are 3-4 times the cost of a packaged Scotch-type boiler.
This horsepower criterion is important because food preparation plants of the poultry, red meat, seafood, and like industries are seldom extremely large. Their power needs are generally within the range of 250-300 horsepower. Most of the power boilers and burners that have been developed for burning fuel wastes require such large dilution ratios, such as 10:1 of sawdust:sludge on dry weight basis for a cascade grate burner, for example, that energy well in excess of this range, such as 800-1,000 horsepower, is produced, and expensive cogeneration facilities are required in order to be able to sell the additional power. The total cost of such an installation is within the range of 1.5 to 2.0 million dollars at the present time. However, the owners of many food preparation plants can not afford to spend such a large amount of money and have no desire to engage in electrical power production.
There is accordingly a need for a process that produces a proportionately smaller quantity of high-energy biomass fuel that is suitable for a gasification-type suspension burner while adequately disposing of the food preparation plant's entire output of wastes. Such a process must be able to dispose of more sludge per generated horsepower and per unit cost of installed facilities than the prior art facilities which are currently available.
Doing so requires utilizing an absorptive bulking agent that is capable of absorbing a relatively large amount of fat from the float sludge in order to minimize dilution of the high Btu value of the fat. It also requires that both the float and waste activated sludges be dried to within a selected range of moisture content in spite of large daily and even hourly fluctuations in wastewater flow rate and in moisture content and fat content of the float sludge.
However, the suspension burner, developed for burning sawdust, can burn such particulate fuels with high efficiency and flexibility and is available in smaller sizes having outputs within the desired range for poultry processing plants. The particle size of such particulate fuels is generally in the range of no more than 1/8 inch.
U.S. Pat. No. 4,565,137 describes a biomass suspension burner for use with furnaces or boilers which has a delivery system for injecting particulate fuel into a combustor, a means for introducing a primary air steam to mix with and conduct fuel into the combustor, and a means for introducing secondary air tangentially to maintain a cyclonic vortex. When the burning, gasified fuel exits the combustor through a nozzle, quartiary air is introduced to burn the gas. Proper flame stability, gasification, and ash fusion are achieved by regulation of the air streams.
U.S. Pat. No. 4,566,393 describes an apparatus having a waste burner for burning woodwaste products, such as wood shavings, wood chips, and sawdust. The burner includes an elongated cylinder having a length substantially larger than its diameter so that the fuel can be heated to a sufficient temperature to pyrolyze volatiles and initiate combustion in the cylinder. A woodwaste burner apparatus receives unburned fuel and combustion products from the burner.
A burner for use in burning granulated solid fuel is disclosed in U.S. Pat. No. 4,574,711. The burner includes a burner head having a combustion chamber and inlet and outlet openings at opposite ends thereof and in communication with the chamber. Granulated solid fuel and primary combustion air are delivered to the inlet opening. The primary air is preheated by movement along the the burner head prior to entering the inlet opening. A retention barrier within the combustion chamber retains fuel particles larger than a predetermined size in the combustion chamber to insure complete incineration thereof.
A particulate waste product combustion system is described in U.S. Pat. No. 4,589,355 which provides controlled incineration of agricultural waste products for utilization of the ash residue and gaseous exhaust.
Other useful apparatuses for burning particulate biomass fuels are described in U.S. Pat. Nos. 4,492,171, 4,671,192, 4,566,393, 4,612,865, 4,538,530, and 4,546,710.
Methods and apparatuses for directly treating sewage sludge to form a fuel therefrom are also available, such as U.S. Pat. No. 4,378,229 which discloses a method for treating sewage in which the combustible components of the sewage are separated from the sewage and utilized as a primary fuel. Separation is done by skimming screened sewage to collect the scum which comprises oils, greases, fats, water, and intermixed solid material. The scum is maintained in a quiescent and substantially nonagitated state for at least twelve hours, during which the combustible oils, greases, fats, and the like are rendered separable from the other components of the scum. Then the fats, oils, greases, and the like are separated from the water and solid material and burned as a primary fuel.
U.S. Pat. No. 4,532,873 discloses a method for preparing hog fuel, other biomass, or peat for efficient burning and heat recovery in a water-wall boiler by drying the fuel to less than 30% moisture content, then pulverizing it to an upper particle size such that all particles will burn in air suspension within the combustion zone and the boiler can meet emission requirements. The pulverizing step is also adjusted so that a fines portion of the fuel is created of such size and in such amount that this fines portion readily self-ignites upon flame initiation, providing sufficient ignition energy that the entire flow of fuel burns without needing conventional fossil fuel support or pilot. A suitable particle size range was found to comprise 65-100% less than 1000 microns and 15-85% less than 150 microns.
U.S. Pat. No. 4,592,291 describes separating sewage sludge from sewage liquid, dewatering and compressing the sludge, subjecting the sludge to microwave radiation to reduce sludge to ash-like residue, and adding the residue back to the sewage liquid.
To prevent plugging of a sludge drying apparatus by the very sticky mass of thickened sludge, U.S. Pat. No. 4,599,954 discloses a process for disposal of wet sewage sludge by admixing a particulate material having Btu values, feeding the mixture to a grinding or milling apparatus in which the particulate material acts to prevent the sewage sludge from caking in the apparatus during the grinding or milling thereof, supplying hot gases to dry the mixture while recirculating a portion of the hot mixture, and disposing of the ground or milled mixture by burning thereof.
If the float and WAS sludges are mixed with sawdust, partially dried to obtain a material that can be spread on farmland, as taught in U.S. Pat. No. 2,861,877, and allowed to remain overnight in the sealed, hot dryer, there may be instant ignition of the gas within the dryer when air is admitted thereto.
If such a partially dried sludge/sawdust mixture is passed through a size-reduction apparatus, such as a hammermill and a screen, a thick, black, gummy material is likely to ooze out of the mixture as it is heated during the size-reduction treatment. If the mixture is fed to a burner after size reduction, such oozing is additionally or alternatively likely to occur there.
Yet when the ratio of biomass material to float sludge (dry basis) is excessively minimized, it has been found by experimentation that a black, shiny, gummy material (almost like tar) is exuded that clogs screens and conveyors before the fuel reaches the burner. Conversely, if the ratio of absorptive bulking agent to float sludge is too great, the burner declines in heat output so that it loses efficiency and can even be incapable of being self sustaining without addition of a support fuel such as natural gas. Similarly, if WAS is entirely substituted for float sludge in such a biomass fuel mixture, the Btu value may be come insufficient in relationship to air input to maintain the flame. On the other hand, if the proportion of float sludge to absorptive bulking agent is too great, the entire burner may be melted down unless the ratio of air input to biomass fuel mixture is suitably and promptly changed.
On the other hand, if 1/8 -inch sawdust, as an exemplary bulking agent, is mixed with a little too much float sludge, it has been discovered that wet and greasy masses of sludge and sawdust, termed "grease balls", are formed within the dryer. These grease balls cannot be easily broken apart and require additional drying time; once they have dried to a certain moisture content, however, they break apart readily.